Refer initially to FIG. 1 which shows a phase loss circuit which is known prior art and such as is used by the assignee of the present invention.
The phase loss circuit of FIG. 1 includes two inputs. One input at terminal B is a summed, three-phase, half-wave rectified voltage proportional to the phase current provided to the motor, or as referred to herein, the instantaneous voltage. In the normal case, the instantaneous voltage is repeating half wave 2.76 milliseconds in duration riding on a DC level as indicated by the graph GX. During a phase loss condition, the instantaneous voltage becomes a repeating half wave but 8.3 milliseconds in duration and dipping all the way to zero level, as indicated in graph G2.
The second input to the phase loss circuit is a peak detected voltage at terminal A which is a smaller proportion of the motor phase current. The peak detected voltage is a fixed proportion of the instantaneous voltage; however, during rapidly changing transients the peak detected voltage reacts or changes more slowly than the instantaneous voltage.
The peak detected voltage and instantaneous voltage are coupled to input terminal pins 1 and 2 respectively of a comparator U2. In the normal case, the instantaneous voltage at pin 2 is riding on some DC level, and the peak detected voltage at pin 1 is something less than the instantaneous voltage. The output at terminal pin 3 of comparator U2 is positive voltage which is blocked by a relatively reversely connected diode D1. During a phase loss condition, the instantaneous voltage will dip to zero; the peak detected voltage will still be a fixed proportion of the instantaneous voltage; and, the output of comparator U2 will pulse negative by each half cycle, as indicated in graph G3, for a length of time as determined by the following equation: EQU Tp=Sin.sup.-1 (Vr/Vp).times.2K
Where:
Tp=Time of the pulse width for two adjacent halfwaves PA0 Vr=Scaled peak detected DC reference PA0 Vp=Peak value of the instantaneous sine wave PA0 K=Conversion factor for changing degrees to microseconds (46.3 usec/degree)
The output pulses from comparator U2 then charge a phase loss capacitor C1 to a tripping level. Thus, a signal from capacitor C1 is coupled through amplifier AMP 1 and diode D2 to motor tripping control.
The method of phase loss sensing is predictable and is used in the prior art. However, the circuit just described will give false indications during starting transients, stopping, and severe load transitions. If the turn-off condition is examined, it is found that the instantaneous voltage will dip to zero and stay there. The peak detected signal will fall to zero some half second after the instantaneous voltage, thereby causing the comparator to switch to its negative output and enabling capacitor C1 to charge to its trip level. This can result in an unwanted or nuisance trip.
Severe load transients can also cause false indication for a short time. The prior art deals with this condition by raising the phase loss trip level so that the amount or level to which the capacitor gets charged during the transition will not trip the circuit. Raising the trip level extends the trip time during a true phase loss condition.